The main problem with multiple dining or shopping hits is that they can be done cheaply. Great for us, but not so great for US, who doesn't want to give 110K for 40 iTunes. If they allowed more hits, they'd probably also introduce a minimum of miles required for it to count, like 30 for a dine. Failing to set limits like this is what burned UA on their Month of Miles and resulted in belated rule changes and a lot of unhappy customers.
The advantage to the current setup is that it's cheap to get into, but hard to master. You can get 3K for free or 10K for a few dollars, but getting to 100K or more requires a minimum of several hundred dollars spend.
Yes, it would be great for it to be even cheaper, but this lessens the incentive for US to keep running it.
The partners might consider promoting it themselves. I was actually surprised to see a banner at Teleflora's site promoting the miles plus one Grand Slam hit. As far as I know, this is the only partner that's mentioned it.